As a cathode active material for lithium-ion battery, a lithium-containing transition metal oxide is generally used. Specifically, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4) or the like is used and compositing these materials has been promoted in order to improve characteristics (enhancement of capacity, cycle characteristics, storage characteristics, decrease of internal resistance, rate characteristics) and in order to enhance safety. Different characteristics are required for the lithium-ion batteries for use in large size applications such as automotive use and load leveling use from characteristics required for the lithium-ion batteries for use in cellular phones and personal computers.
The cycle characteristics of a battery, which are good, are beneficial in that decrease of the battery capacity caused by charge-discharge of the lithium-ion battery is small, and therefore life thereof is prolonged. In recent years, the cycle characteristics have been known to have close relation to generation of crack of the cathode active material particles associated with charge-discharge of the battery (Non Patent Literature 1). The generation of crack of the cathode active material particles causes increase in the resistance within the electrode and consumption of Li due to a side reaction thereof associated with the increased specific surface area, thereby resulting in lowered capacity at discharge.
To such a problem, Patent Literature 1, for example, discloses a lithium secondary battery comprising a substance, as a cathode active material, which contains a lithium-transition metal composite oxide expressed by general formula: LiXMYOZ-δ, wherein M represents Co or Ni being a transition metal element and relationships (X/Y)=0.98 to 1.02 and (δ/Z)≦0.03 are satisfied, and also vanadium (V) and/or boron (B) in an amount of ((V+B)/M)=0.001 to 0.05 (molar ratio) based on the transition metal element (M) constituting the lithium transition metal composite oxide, the substance having a primary particle diameter of 1 μm or more, a crystallite size of 450 Å or more and a lattice distortion of 0.05% or less. Patent Literature 1 states that a lithium secondary battery can be provided, which has low internal resistance, high output and high capacity and also exhibits excellent charge-discharge cycle characteristics even under a high-temperature condition.
In addition, Patent Literature 2 discloses a nonaqueous solvent secondary battery employing a hexagonal crystal LiCoO2 as the cathode active material, the LiCoO2 crystal having a lattice constant of the c axis of 14.05 to 14.15 Å and a crystallite size in the (110) surface direction of 350 to 400 Å and that in the (003) surface direction of 260 to 300 Å. Patent Literature 2 states that a nonaqueous solvent secondary battery can be provided, which has reduced capacity deterioration caused by charge-discharge cycles.
Further, Patent Literature 3 discloses a fluorine-containing lithium-cobalt based composite oxide which has a crystallite size in the (110) vector direction of 950 Å or less, an amount of soluble fluorine atoms of 2000 ppm or less, and an F/Co atomic ratio of 0.01 or less. Patent Literature 3 states that the cycle characteristics of a lithium-ion secondary battery are improved.